Synthesis gas is a mixture which includes carbon monoxide (CO) and hydrogen (H2). Synthesis gas is typically produced by one of two processes, either from solid feedstocks, such as coal, by gasification with oxygen and steam, or from gaseous feedstocks, such as natural gas, by reforming with oxygen (known as partial oxidation reforming) or water (known as steam reforming). A combination of partial oxidation and steam reforming, namely autothermal reforming, is also commonly applied. The oxygen required for the production of synthesis gas is usually obtained from air using conventional cryogenic air separation technology. The synthesis gas produced is used to produce a wide range of carbon based chemicals, e.g. methanol and liquid hydrocarbons via Fischer-Tropsch synthesis.
Synthesis gas production processes are energy intensive and contribute significantly to carbon dioxide emissions. Carbon dioxide is a major greenhouse gas, and its emission into the atmosphere is not environmentally friendly.
The problem of carbon dioxide emissions can be dealt with in various ways e.g. by carbon dioxide capture and sequestration, reduction of carbon dioxide emissions via improvement of thermal efficiency and substitution of conventional carbon based power and heat generation facilities with a non-carbon source, e.g. nuclear energy.
Synthesis gas production processes operate at elevated temperatures and, depending on the type of technology used to generate the synthesis gas, can produce a hot synthesis gas at a temperature above 900° C. Heat is typically recovered from the hot synthesis gas using waste heat boilers producing steam. This steam is typically used to drive steam turbines for cryogenic air separation units and/or to produce electrical power. It is important to note that conventional cryogenic air separation processes consume significant quantities of power. Heat recovery using waste heat boilers also contribute considerably to second law thermodynamic losses in processes producing the synthesis gas due to large temperature difference driving forces used in such waste heat boilers. In other words, the use of waste heat boilers downgrades high quality or high temperature heat to a lower quality or lower temperature heat which is undesirable, as heat at a higher temperature can be used to produce more power compared to the same amount of heat at a lower temperature. High temperature difference driving forces reduce overall thermal efficiency of a process and therefore potentially worsens the problem of carbon dioxide emissions.
One way to reduce large temperature difference driving forces in waste heat boilers would be to raise the steam pressure or to superheat the steam. However, the fact that the critical temperature of water is 374° C. places an upper limit on the temperature at which saturated steam can be produced in waste heat boilers. Also, when using steam to generate power in e.g. a Rankine cycle, steam is typically not superheated to temperatures above 565° C. because of material of construction considerations.
Attempts to reduce carbon dioxide emissions via thermal efficiency improvements should therefore focus on addressing the problem of high temperature difference driving forces and also on reducing the power consumption of cryogenic air separation processes. However, since cryogenic air separation is a mature technology, only incremental reductions in cost and power consumption are expected. An alternative process for separating oxygen from air is the use of Ion Transport Membranes (ITM's). The ITM oxygen process uses ceramic membranes operated at high temperature (typically 760-930° C.) to separate the oxygen from air. It is believed that the ITM oxygen technology could significantly lower the cost of oxygen production. This high temperature oxygen-producing process lends itself to integration with processes wherein oxygen, power and steam are required. In an ITM oxygen process ceramic membranes separate oxygen from air at high temperature in an electrochemically driven process. The oxygen in the air is ionized on an upstream surface of the ceramic and diffuses through the membrane as oxygen ions driven by an oxygen partial pressure gradient, forming oxygen molecules on a downstream side of the membrane. The ITM oxygen process produces a hot, substantially pure oxygen stream or permeate stream and a hot, pressurised oxygen-depleted stream or reject stream from which significant amounts of energy can be extracted. The effective use of this energy in the overall operation of an ITM oxygen process is necessary for the system to be competitive with conventional cryogenic air separation technology. The energy recovery and effective use thereof are possible by integration of compressors, gas turbines, hot gas expanders, steam turbines and heat exchangers with the membrane module.
Research and development on nuclear-assisted synthesis gas generation processes have thus far attempted to match the synthesis gas generation process operating temperature with the highest temperature heat that can be made available from a nuclear reactor loop. High temperature gas cooled nuclear reactors are able to provide heat at temperatures of about 750-950° C. At these comparatively low temperatures, reasonable synthesis gas generation process options are limited, especially when a gasification process is employed.
Synthesis gas generation processes typically form part of large-scale facilities producing carbon-based chemicals. Such facilities typically include further processing steps operating at temperatures below 800° C. or even more typically below 500° C. Although these further processing steps may be promising candidates for heat integration with nuclear heat sources, it was found that these further processing steps are also promising candidates for heat integration with hot synthesis gas produced in a synthesis gas generation process. It has also been found that in such facilities at temperatures below about 250° C. there typically is a number of sources and sinks of heat, with the heat sources becoming numerous with decreasing temperature. There is thus typically an excess of available lower grade heat. Consequently there is little incentive to rather provide low grade heat from a nuclear source. A more conventional light-water nuclear reactor would probably be the preferred choice for supplying low grade heat. There is thus a perceived lack of opportunities for integrating a nuclear heat source with large-scale facilities producing carbon-based chemicals, and particularly so for integrating a nuclear heat source with a synthesis gas generation process. This has led to significantly different strategies for using nuclear energy, most notably nuclear driven hydrogen production through water splitting. Embodiments of the present invention in contrast propose a new and different approach.